1. Field of the Invention
The invention relates generally to the field of weighing, and specifically to measurement of force or weight in situations where the location of the force or weight is not known and/or cannot be controlled. This is especially relevant to weigh-in-motion or mobile applications such as fork trucks and refuse collection vehicles where the location of the center of the load is unknown and the ground or floor may not be level. In this application, the load is contained in some large bin or atop a pallet, and is acquired by the vehicle via a “fork” comprised of a cantilever beam that is inserted into receptacles in the bin or under the pallet.
2. Description of Related Art
Devices for measuring weight or force are common in everyday life and commerce. Often these devices function by measuring the mechanical strain in a small area of the device with an attached resistor commonly referred to as a “strain gauge” which changes resistance with strain or by balancing against an accurately known reference weight on a lever device. In these devices the location of the weight must be controlled or the effect of the weight on the loading platform must be controlled mechanically. In some devices, where position of the weight cannot be precisely controlled or known, such as scales for use in refuse truck front forks, the designers have opted to use a shear force measuring load cell or to place strain gauges in a location and in geometric features optimized to measure shear force near the root of the cantilever beam which comprises the lifting fork. Theoretically, the shear stresses anywhere in the lifting fork between the proximal end of the beam (the end where the beam is attached to the vehicle) and the load are a function of the applied load and geometry and are independent of the exact location of the loading of the beam. This means is described in patent (Ruge U.S. Pat. No. 2,597,751). In practice this has been difficult and resulted in cantilever beam scales which contain mechanical complications, produce inaccurate results, and are fragile. Further, replacement of the strain gauges or retrofit of a gauged system to a non-weighing one requires removal of the fork. Further, the “shear” type load sensors cannot compensate for off-level loading condition by themselves. The shear sensing beams must also include weakening features such as cutouts (described in Ruge and shown here as typically practiced) due to the fact that the shear forces, while constant over some length of the beam, are small compared to the bending forces. The bending forces in the cantilever beam are generally greater, but vary relative to the position of the weight or load and thus in prior art, cannot be used to measure a weight in devices that cannot control or determine location of the load. What is needed therefore is a means to resolve a weight or force without knowing or controlling it's location in the bin, on the pallet or knowing or controlling the angle of the approach (off-level condition of the ground) using a mechanically simple and robust, easily serviced and retro-fittable device. This device should use the greater bending forces present in the lifting fork instead of the shear forces.